The manufacture of aqueous polysiloxane emulsions is a widely practiced art typically resulting in aqueous dispersions of broad particle size distributions. These dispersions have a broad utility, including the personal care, adhesives, and coating industries.
Current manufacturing practices encompass emulsification, into water, of pre-existing polysiloxane fluids and/or gums via standard high shear homogenization techniques to directly yield the dispersions. Alternatively, it is known to batch stir polysiloxane precursors in water in the presence of surfactants, under low shear non- homogenizing conditions, subsequent to thermal emulsion polymerization to yield the polysiloxane dispersion. These processes yield a dispersion having a broad particle size distribution. As in many areas of technology such as coatings, adhesives, inks, personal care and the like, which utilize polymer particles, particle size control can be critical to attain optimum performance of the material in its end use application. This is also true for polysiloxane emulsions where particle size has become increasingly important, for example in designing dispersions for the personal care industry.
Examples of polysiloxane emulsion polymerization processes have been known. Base catalyzed emulsion polymerization is disclosed in U.S. Pat. No. 2,891,920, which describes the process as base catalyzed with a cationic surfactant. European patent application EP 459500 A2 911204 also discloses a base catalyzed emulsion polymerization process. A base catalyzed emulsion polymerization process is thus described to comprise a process proceeding via an ionic mechanism involving pre-emulsifying cyclosiloxanes into water in the presence of a cationic surfactant and a source of hydroxide ion. These base catalyzed emulsion polymerization processes can yield high molecular weights, however reaction times are very long.
Acid catalyzed emulsion polymerization of siloxanes is also known. Journal of Polymer Science, Part C (27), 27, 1969 discusses the use of dodecylbenzenesulfonic acid as a catalyst and surfactant for the synthesis of polydimethyl-siloxane emulsions from permethylcyclosiloxanes.
Semi-continuous processes for polysiloxane emulsions are described in Japanese patent application JP62141029 A2 870624. This process utilizes a continuous addition of a pre-emulsion of cyclosiloxanes precursor to a reactor containing large amounts of acid catalyst emulsifier. Particle sizes of polysiloxane emulsions thus formed are normally restricted to sizes below 150 nm.
Given a variety of processes disclosed for the production of polysiloxane emulsions, it is surprising that emulsion processes are still being sought which will allow simultaneous control of polysiloxane molecular weight as well as the dispersion particle size within a narrow range. It is also surprising that a process for acid catalyzed synthesis of high molecular weight pure polysiloxane emulsions having a particle size in the range of 50 nm to a few microns is not available. Polysiloxane emulsions having such a particle size in a desired narrow range i.e., mono-modal particles, can provide improved polysiloxane deposition and film formation in skin care products, and would also allow the manufacture of very high emulsion solids, for example, up to 75% by weight of high molecular weight polysiloxanes, at workable emulsion viscosities. Strict control of particle size and distribution would allow specific tailoring of the dispersion to the application, with a resulting increase in performance efficiency.
Publications that relate to polysiloxane emulsion technology surprisingly do not provide a process to make essentially mono-modal, or mono-modal and multi-layered particles. Mono-modal multi-layered particles can provide dispersions with multiple properties. Thus for example, different properties can be imparted to each layer of the particles in a polysiloxane emulsion.
There is thus a need to provide polysiloxane emulsions with improved particle size control in the particle size range of about 50 nm to a few microns. There is also a need to provide a process to make mono-modal polysiloxane emulsions having a predetermined particle size in a narrow range within the aforementioned particle size range. There is also a need to provide polysiloxane dispersions containing particles possessing multi-layered structures, and a process for making such multi-layered particles.